[TowerTalk] element length criticality

[Jim Lux]

On 4/14/15 5:44 AM, Peter Voelpel wrote:> Why is a difference of a 1/16" 
of element length a disaster with an antenna
 > to cover 430-438MHz?
 >

a long boom Yagi with many elements has fairly critical lengths to get 
the side and back lobes down.  Forward gain isn't strongly affected, though.

10 MHz BW is 1%.

1/16" (actually 0.125" total element length) out of a 12" element is also 1%

If you're looking for -20dB or -30dB side/backlobes then 1% kinds of 
errors can be a problem.

These kinds of designs are optimized for one frequency (432, usually) 
and while they might show decent feedpoint match for a fairly wide 
frequency, their pattern probably isn't very good 5MHz away from the 
design frequency.


Element length in long boom yagis is like surface smoothness in a dish. 
  Huge dings in a dish don't change the gain much, but boy, do they 
create sidelobes.